A brake pad is an important constituent of a brake system for various transportation means equipped with a drum brake or a disc brake. The working principle of the brake is mainly based on rubbing, i.e. using the rubbing between a brake pad and a brake disc (drum) and between a tire and the ground, to convert the kinetic energy of vehicle movement to thermal energy through the rubbing, and to make the vehicle stop. A brake pad generally consists of a steel plate, an adhesive thermal insulating layer and a rubbing block. The rubbing block is pressed on the brake disc or a brake drum producing friction during brake operation, to slow down the vehicle. Due to the rubbing effect, the rubbing block would gradually wear; and when it wears to a critical position, the brake pad must be replaced, otherwise, the braking effect would decrease and even cause safety accidents.
Generally, whether the brake pad needs to be replaced is judged through two manners as follows: 1. The judgement is based on observation of the wearing situation of the brake pad: the thickness of a new vehicle brake pad is generally about 14 mm, which gradually gets thinner during continuous rubbing process. Professional technicists recommend that when the thickness of the brake pad is only half (about 7 mm) of the original thickness, observing by naked eyes, the vehicle owner should increase the self-checking frequency, and be prepared to replace the brake pad at any time. 2. The judgement is based on sound: the steel plates of some brake pads have protruding portions (a metal plate is additionally added on the brake pad in some cases) as a critical indicator, whose top point is related to the critical thickness of the brake pad. When the critical indicator touches the brake disc, it means that the thickness of the brake pad has reached or exceeded the critical thickness. When the brake is slightly stepped with a hiss, the brake pad must be replaced immediately. The brake disc also should be inspected immediately after the hissing to determine whether it should be replaced, because bad damage may occur.
The two judging methods mentioned above both have flaws. Observing with naked eyes has relatively large error because it is difficult to measure the thickness directly after the installment of the brake pad. Therefore, observation can be used only for preliminary judgment. Whether the brake pad needs to be replaced should be determined by detaching and measuring the brake pad, which is time consuming. Judging by sound is relatively precise but costs more. Because the hissing means the brake pad should be replaced.